Objects have a natural tendency to keep doing whatever they're already doing. If a hockey puck is sitting on the ice, it will stay there forever unless someone hits it. If it's already sliding across frictionless ice, it will keep sliding at the same speed and in the same direction unless something stops it or changes its path. This fundamental principle is Newton's First Law of Motion.

This resistance to a change in motion is a property of matter called inertia.

Every object has inertia. But some objects have more than others. The amount of inertia an object has is determined by its mass. Mass is, in essence, a measure of how much stuff an object is made of. A bowling ball has much more mass than a tennis ball, and therefore it has more inertia. You can easily change the tennis ball's state of motion with a light nudge, but you need to apply a much greater push to get the bowling ball rolling.

This brings us to the crucial part of the law: the role of forces. A force is a push or a pull. The law doesn't say that an object's motion can't change. It says it won't change unless a force makes it happen.

Imagine a book resting on a table. Gravity is pulling it down, and the table is pushing it up with an equal and opposite force. These forces are balanced. The net force, or the sum of all forces acting on the book, is zero. Because the forces cancel each other out, the book's state of motion doesn't change. It remains at rest, just as Newton's First Law predicts.

Now, if you push the book from the side, you introduce a new, unbalanced force. If your push is stronger than the force of friction between the book and the table, the net force is no longer zero. The book will accelerate and start to move. Only an unbalanced force can change an object's velocity.

This concept is at play all around us. When you're in a car traveling at a constant speed, both you and the car are in a state of uniform motion. If the driver suddenly hits the brakes, the car rapidly slows down due to the braking force. However, there is no force acting on you to slow you down. Because of your inertia, your body continues to move forward at the original speed until an unbalanced force, like the seatbelt, stops you.

Understanding this law is the first step in analyzing the motion of objects. It establishes the baseline condition: constant velocity. Any deviation from this baseline, meaning any acceleration, must be caused by a net force.

This first law sets the stage for the next two, which describe exactly how forces and mass are related to changes in motion.